A typical digital to analog signal converter apparatus is comprised of a monolithic or single digital to analog converter device, wherein a digital signal is input and a variable analog output signal is provided in response thereto. Such devices are typically electronic integrated circuit chips and may be utilized in microprocessor-based control devices to control apparatus which require an analog voltage input. The digital to analog signal conversion is necessary in this situation due to the fact that the output of the typical controlling microprocessor is in the form of digital signal unintelligible to an analog apparatus or device.
Typically, a single microprocessor-based controller will desirably control several analog devices to minimize the cost of the controller. However, because of the relative expense of the monolithic digital to analog signal converter chip, it is undesirable to provide for each analog device an individual monolithic digital to analog converter chip. Therefore, numerous approaches have been taken to reducing the expense of devices incorporating such monolithic digital to analog converter chips. The various approaches have included the provision of multiplexer circuitry, sample and hold circuitry, and opamp circuitry. These solutions typically provide that the monolithic digital to analog converter chip handles a plurality of analog outputs by sampling the digital signal directed to each output and fixing an analog output voltage which is held until the monolithic digital to analog converter again is available to sample and adjust that analog output. While these approaches eliminate the need for multiple monolithic digital to analog signal converter chips, they require the incorporation of a relatively large number of components, while rendering impossible the continuous control of the analog output for each analog output for each analog device being controlled by the microprocessor.
Therefore, it has been necessary to provide either a number of relatively expensive monolithic digital to analog converter chips to maintain continuous control of each analog device, or, alternatively, to provide only a single monolithic digital to analog converter chip and utilize one of a variety of relatively complex switching circuits to provide partial, intermittent control of the analog signal output.
Furthermore, typical digital to analog signal converters provide an arbitrary analog voltage output signal when the monolithic digital to analog signal converter is powered up. Therefore it is often necessary to provide safeguards in the form of additional circuitry in the controller in order to prevent any such arbitrary signals. An arbitrary output signal from the control device would cause the analog device to respond in an arbitrary fashion with unpredictable and possibly even unsafe results.
Thus, it is an object of the invention to provide a digital to analog signal converter having the capability for continuous monitoring of the analog output signal.
It is an object of the invention to provide such a digital to analog signal converter device which has a minimum of circuitry components.
Another object of the invention is to provide a digital to analog signal converter device utilizing relatively inexpensive circuitry components.
Yet another object of the invention is to provide a digital to analog signal converter device which will be simple to manufacture and maintain.
Yet a further object of the invention is to provide a digital to analog converter device which provides a zero output upon the initial power-up condition to eliminate the necessity of expensive safeguard circuitry.
A still further object of the invention is to provide a digital to analog signal converter device suitable for an inexpensive and simple application to a microprocessor operated device controlling a plurality of analog devices.
These and other objects of the invention will become apparent from the attached drawing and the description of the preferred embodiment that follows hereinafter.